This invention relates to a thermal sensor for detecting the overheating of lighting fixtures, and more particularly to a heat flow detector for use in conjunction with a recessed incandescent fixture in which a dimmer is provided to control the intensity of the light.
Lighting fixtures are subject to overheating and produce dangerous conditions to the surrounding structure. This is especially a problem with recessed fixtures placed above a ceiling where there is often inadequate ventilation. The space above the ceiling is generally filled with thermal insulation which limits the flow of air in to the space surrounding the lighting fixture. Installing barriers around the recessed fixtures is prohibitively expensive and difficult to achieve. While in new building structures some possibilities exist for spacing the insulation from the recessed fixtures to provide for dissipation of the heat in the surrounding area, in existing homes and buildings this may not always be possible. As a result, the heat generated within the fixture cannot sufficiently dissipate and overheating of the lamp occurs.
Overheating of the lamp can also occur as a result of overlamping, whereby a lamp of greater wattage then the fixture is rated for, is utilized within the lamp. With the larger wattage lamp, the heat generated by the lamp is not dissipated quickly enough and the fixture temperature becomes higher than intended by the fixture designer.
Overheating of the lamp either due to overlamping or inadequate spacing from the thermal insulation can result in damage to the lamp and its parts. Generally, a particular fixture is designed of plastic parts and wire insulation which is adequate for use with predesigned rating of heat. Overheating of the lamp results in damage to the fixture. Even worse, eventual deterioration of the fixture components may actually result in fire. Such fires from overheated fixtures have caused millions of dollars of property damage.
The use of thermal protectors for various types of circuits have previously been known. In most cases, thermal protectors have been utilized for motor circuits. By way of example, U.S. Pat. No. 3,141,996 describes a thermal protector which is placed in heat exchange relationship with a motor so that overheating of the motor will actuate the thermal switch and disconnect the motor from operation. Similarly, U.S. Pat. No. 4,136,323 describes an overload circuit for a motor winding, whereby the overload circuit is in direct contact with the windings such that occurrence of an over-temperature condition in the windings actuates the protector circuit. Thermal protectors have also been used with lighting fixtures such as described in U.S. Pat. No. 4,131,868. In this invention, the socket of the incandescent lamp includes a thermal protector in direct contact with the socket, whereupon overheating of the socket causes activation of the thermal protector. Likewise, U.S. Pat. No. 4,388,677 describes a housing for a lamp which includes a built in thermal protector.
While these devices are useful for the electrical equipment for which they are designed, it is not always possible to build in the thermal detector directly as part of the electrical apparatus as in these inventions. Accordingly, co-pending Ser. No. 433,579, assigned to the assignee of the present invention, describes a heat flow detector which is not part of the lighting fixture itself. The heat flow detector is placed in a junction box which is located to the lighting fixture. The heat flow detector responds to the same environment of the fixture in which the lighting fixture is located. Accordingly, any restriction of the flow of heat in the environment of the fixture also restricts the flow of heat from the junction box. The heat flow detector includes a thermal switch which is serially connected with the lighting fixture to disconnect the lighting fixture from the source of energy upon actuation of the switch. A heating element is thermally in contact with the switch. The heating element is designed to provide heat above that of the surrounding environment.
When the lamp overheats, the heat within the environment of the lamp increases. Dissipation of the heat is restricted by the thermal insulation surrounding the lamp. Since the junction box is within the same environment as the lamp, the temperature in the surrounding environment adjacent the junction box also increases. The heating element is designed so that its heat in excess of the temperature of the surrounding environment produced by an overheated lamp will be sufficient to actuate the switch. The switch will then disconnect the lamp fixture.
While such heat flow detector for a recessed incandescent fixture is useful, various electrical codes require that the thermally protected incandescent lighting fixture also operate when a dimmer is utilized to control the fixture. In the aforementioned co-pending application, the heating element is energized by the voltage from the same source feeding the fixture. With the presence of a dimmer, the voltage across the lighting fixture can be lowered. Similarly, the voltage across the heating element will be reduced and possibly below the required operating voltage. The voltage will therefore not be sufficient for the heating element to reach its operating temperature. As a result, with the presence of a dimmer, the aforementioned heat flow detector may not provide adequate biasing heating to oeprate the thermal switch.